Marlene,
Not sure you can do this within screen tips, but you can fake it.
1. Duplicate existing "launch" slide
2. On duplicate slide add your photo as it might appear in screen tips
3. Hide this slide
4. Create a Custom Show from this duplicate slide (with the photo)
5. At origininal slide select the object from which your photo will be
launched
6. Right click and create an action setting.
7. Hyperlink the action button to the Custom Show you just created.
8. Before for you close out of the action settings dialog be sure to check
off Show and Return.
Now when you click on the object in the original slide, the photo will "pop
up." When you advance the pop up, you will return to the original slide.
Hope this helps.
Sandy